It was probably a surprise to you both
You could be my husband posting! He felt (& sometimes may still feel) the exact same way. So sorry to hear those feelings are still prevalent because of course you're right, the relationship is not what you envisioned when you married and is now something that you never even entertained as a possibility.
Speaking as the "kicked puppy"; she's feeling massive guilt for "hi-jacking" the marriage into a different type of relationship. She LOVES you and feels awful for hurting you. She's also trying to be very true & honest to herself and you. Chances are it was a shock to her as well that she could so powerfully love more than one person. But she does and she wants to express, feel and share that love and she doesn't want anyone to hurt. She's so grateful right now that you feel it's worth the hardship & pain to stay in the marriage & make it work.
You must be an incredible man to fight for this, to feel the pain and do it anyway. It takes a REAL man to endure all this and love her through all the changes. Wish there was a magic wand to help with your resentment, it's understandable and you are so very entitled to feel that way. Perhaps focus on the love? In our case & it seems others mentioned in this site; it does get easier, better with time & practice.
OH! & get some hobbies, buddies, sex partners; SOMETHING to help "distract" you during the tougher times. That's what seems to have worked best for my husband. He's not (yet?) found the same kind of connection as my bf & me, but he has found some really lovely ladies that have been wonderful FWB type additions to his life, & he's turned his frustration & anger toward working out; been a great stress relief.
HUGS! Hang in there.